Liquefaction of natural gas employing cascade refrigeration



April 29, 1969 1. A. PRYOR ET AL LIQUEFAGTION OF NATURAL GAS EMPLOYING CASCADE REFRIGERATION v Filed Feb. 11. 1966 I of 5 Sheet INVENTORS JOHN A. PRYOR CARL A. BOLEZ ATTORNEY LIQUEFACTION OF NATURAL GAS 'EMPLOYING CASCADE REFRIGERATION Sheet Z of 3 J. A. PRYOR ET April 29, 1969 Filed Feb. 11, 1966 April 29, 1969 A, PRYOR ET AL 3,440,828

LIQUEFACTION OF NATURAL GAS EMPLOYING CASCADE REFRIGERATION Filed Feb. 11, 1966 395m 263m Na United States Patent Ofice U.S. C]. 62-27 13 Claims ABSTRACT OF THE DISCLOSURE Natural gas is liquified and subcooled by passing in heat exchange with a plurality of separate refrigerants which are at progressively decreasing temperature levels and in cascade relationship. At least one of the refrigerants is provided at a single pressure level while a plurality of refrigerants are provided at at least three progressively decreasing pressure levels. The liquified natural gas is cooled below its bubble at a low pressure and reduced in pressure to said low pressure while the liquified natural gas is maintained below its bubble point. Liquified natural gas at the low pressure is stripped of nitrogen. Natural gas at high pressure is stripped of hexane in an initial step of the process.

This invention relates to improvements in the liquefaction of gaseous mixture-s for storage under relatively low pressure.

Although the present invention has particular utility and is described in the environment of liquefaction of natural gas, it is to be expressly understood that the principles of the present invention may be employed in whole or in part in low temperature processes for effecting liquefaction of gases as well as gaseous mixtures in addition to natural gas.

The storage of natural gas in liquid phase under relatively low pressure is of continuing importance. A major application occurs in areas fed by natural gas through pipelines in an attempt to meet demand during peak periods of consumption. It has been proposed in the past to liquefy for storage under relatively low pressure natural gas delivered by the pipeline during periods of low consumption and, when the demand is high, to vaporize the liquified natural gas to supplement the natural gas supplied by the pipeline. The feasilbilty of such a system depends upon economic considerations and the power required to effect liquefaction of natural gas for storage under low pressure is of primary importance, if not the controlling factor, in determining feasibility of the system under any circumstances.

It is accordingly an object of the present invention to provide a novel process for liquefaction of gaseous mixtures for storage under relatively low pressure.

Another object is to provide a novel process for liquefaction of gaseous mixtures for storage under rela tively low pressure which is capable of effecting the liquefaction with expenditure of low power as compared to prior processes.

Still another object of the present invention is to provide a novel process for liquefaction of gaseous mixtures for storage under relatively low pres-sure in which the gaseous mixture is subjected to selective separation to 3,440,828 Patented Apr. 29, 1969 remove undesirable components from the gaseous mixture prior to storage.

Still another object of the present invention is to provide a novel process of the foregoing character in which the separation of undesirable components of the gaseous mixture is achieved by novel arrangements with minimum increase in capiatl investment and operating costs.

Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings which disclose a preferred embodiment of the invention. It is to be expressly understood, however, that the drawings are designed for purposes of illustration only and not as a definition of the limits of the invention, reference for the latter purpose being had to the appended claims.

In the liquefaction process provided by the present in vention, compressed gaseous mixture to be liquefied is treated in a conventional way to remove carbon dioxide and water vapor and other impurities such as hydrogen sulfide and is then successively passed in heat interchange with a plurality of refrigerants establishing progressively decreasing temperature levels to effect liquefaction of the gaseous mixture and subcooling of the liquefied gaseous mixture to permit reduction in pressure of the gaseous mixture for storage under relatively low pressure with minimum loss of liquefied material by flashing. In accordance with the present invention, the plurality of refrigerants at progressively decreasing temperature levels are provided by a novel refrigeration system of the cascade type in which the refrigerants of each stage of the cascade system are provided at a single temperature level or at multiple temperature levels in such a manner as to reduce irreversibility losses consistent 'with capital investment and operating costs to obtain optimum efficiency. The present invention also obtains selective removal of undesirable components of the gaseous mixture during the process of liquefaction. In the liquefaction of natural gas, it is desirable to remove the nitrogen component prior to storage. In systems provided heretofore, nitrogen is removed from liquefied natural gas prior to its storage by the use of complicated fractionating columns requiring reflux condensers which materially add to the operating costs of the process. The present invention achieves the same result from a simple stripping operation which is made possible by a novel concept of and a novel arrangement for deeply subcooling liquefied natural gas containing the nitrogen component so as to permit stripping of the nitrogen from the liquefied mixture under reduced pressure without uneconomical loss of any desirable components of the gaseous mixture alone or in combination with the further concept of providing the stripping gas by cooling the feed mixture by vaporizing nitrogen-free liquefied natural gas. The novel arrangement for effecting such deep subcooling of the liquefied gaseous mixture containing the nitrogen component is achieved, at least in part, by the novel arrangement of the present invention of providing plural zones of refrigerant at progressively decreasing temperature levels for effecting liquefaction and deep subcooling of the natural gas.

The present invention also provides a novel natural gas liquefaction process in which the hexane content of the liquefied gas is maintained below a critical value above which malfunctions of the process will occur. Although hexane is soluble in liquefied natural gas, it has been determined that, when the hexane content exceeds the solubility limit, the hexane will crystallize at low temperatures involved in natural gas liquefaction processes, causing malfunctions or eventual stoppage of the process.

In accordance with the principles of the present invention, the compressed natural gas prior to its complete liquefaction is subjected to a novel treatment to remove substantially the total hexane content or at least to reduce the hexane content to below the saturation limit which is approximately 0.1% of the total mixture. The hexane removal is accomplished in a novel manner employing relatively simple apparatus which does not materially increase the operating costs of the process, and the removed hexane is passed in heat interchange with the compressed natural gas feed of the process to recover its refrigeration content, and is thereafter employed as fuel gas to provide a part of the power requirements of the process.

FIGURES 1A, 1B and 1C of the drawings diagrammatically disclose a low temperature process embodying the principles of the present invention.

With reference to FIGURE 1A of the drawings, a stream of natural gas such as derived from a pipeline enters the cycle through conduit 10 under pipeline pressure of about 450 p.s.i.a. The natural gas may be of the following composition:

Component: Mol percent 91.30 C 2.49 C 0.67 0 0.32 C 0.11 0 0.05 N 2.99 CO 1.99

The pressure of the natural gas is raised by compressor 11 to about 550 p.s.i.a. and then passed by conduit 12 to carbon dioxide removal device 13 which functions to remove carbon dioxide and any hydrogen sulfide that may be present by an amine extraction process or other arrangement suitable for that purpose. The carbon dioxideifree natural gas stream is then conducted by conduit 14 for flow through coil 15 of heat exchange device 16 in heat interchange with liquid glycol 17 at a temperature of about 55 F. In the preferred mode of operation, the liquid glycol 17 is maintained at the lowest possible temperature without formation of hydrates in the natural gas stream. The compressed stream of natural gas leaves the heat exchange device 16 at about 60 F. and is passed by conduit 18 to driers 19 for removal of moisture. The driers may be of any conventional construction such as vessels filled with activated alumina or molecular sieve material and arranged in parallel to be switched alternately off-stream for reactivation. The moisture-free natural gas stream leaves the driers 19 by conduit 20 and a major portion of the stream comprising about 93% of the total natural gas entering the cycle is conducted by conduits 21 and 22 for flow through coil 23 immersed in a pool 24 of liquid propane contained within a vessel 25. The liquid propane is at a temperature of about 7 F. and the natural gas flowing from the coil 23 to conduit 26 is cooled to about 2 F. A small portion of the natural gas stream leaving the driers 19, comprising about 6% of the total feed to the cycle, is branched from the conduit 20 at point 27 and conducted by conduit 28 for heat interchange with a relatively cold refrigerant of the cycle, described below, to cool the substream to about 'l23 F., and the thus cooled substream is conducted by conduit 29 for merger with the cooled major portion of the natural gas stream in conduit 26 at point 30. The remaining portion of the natural gas stream leaving the driers 19, comprising about 1% of the total feed to the cycle, is branched from the conduit 21 at point 31 and conducted by conduit 32 for fiow through passageway 33 of heat exchange device 34 in heat interchange with relatively cold fluid, described below, to cool the relatively small substream to about 54 F. From the heat exchange device 34, the cooled substream of natural gas is passed by conduit 35 for merger at point 30 with the cooled natural gas in conduits 26 and 29 and the total natural gas feed at a temperature of about 13 F. is fed by conduit 36 to the base of a stripping column 37.

The column 37 provides a stripping zone containing liquid vapor contact means such as conventional bubbletype fractionating trays 38 and operates under a pressure of about 540 p.s.i.a. to produce a liquid fraction collecting in a pool 39 in the base of the column and a vapor fraction withdrawn from the top of the column through conduit 40. As shown on FIGURE 1B, the conduit 40 conducts the vapor fraction for flow through coil 41 immersed in a pool 42 of liquid ethylene at a temperature of about 64 F. contained within a vessel 43. The vapor fraction is cooled to about --61 F. and partially liquefied upon flowing through the coil 41 and then passed by conduit 44 to phase separator 45 wherein the liquefied portion collects in a pool 46 and the vapor portion is withdrawn through conduit 47. The liquefied portion is withdrawn from the phase separator 45 through conduit 48 and is introduced thereby into the stripping column 37 above the uppermost fractionating tray. As described in detail below, the liquid collecting in the pool 39 in the bottom of the column contains substantially the total hexane content of the incoming natural gas stream and such liquid is passed by conduit 49 to pressure reducing valve 50 and then to the shell side 51 of the heat exchange device 34 to effect cooling of the substream of natural gas feed described above. The liquid containing hexane is at least vaporized in the heat exchange device 34 and is withdrawn from the cycle by conduit 52 at substantially ambient temperature for use as fuel gas.

The vapor withdrawn from the phase separator 45 by conduit 47, being substantially hexane-free and comprising substantially the total natural gas fed to the cycle, is flowed through coil 55 immersed in a pool 56 of liquid ethylene at a temperature of about 112 F. contained within vessel 57. The natural gas stream leaves the coil 55 at about 107 F., partly in liquid phase, and is conducted by conduit 58 for flow through coil 59 immersed in a pool 60 of liquid ethylene at a temperature of about 143 F. contained within vessel 61. The natural gas stream, now at a pressure of about 500 p.s.i.a., is totally liquefied upon flowing through the coil 59 and leaves the coil 59 by way of conduit 62 at a temperature of about -l35 F.

As shown on FIGURE 1C, the conduit 62 conducts the liquefied natural gas stream for flow through coil 63 immersed in a pool 64 of liquid methane at a temperature of about l95 F. contained within vessel 65. The liquid natural gas stream is further cooled to about 190 F. upon flowing through the coil 63 and is then conducted by conduit 66 for flow through coil 67 immersed in a pool 68 of liquid methane at a temperature of about -225 F. contained within vessel 69 to effect further cooling of the liquid natural gas to about 200 F. The liquid natural gas, now at a pressure of about 480 p.s.i.a., is then conducted by conduit 70 for flow through boiling coil 71 of stripping column 72 where the liquid natural gas is further cooled to about 225 F. upon heat exchange with liquid collecting in a pool 73 at the bottom of the column. From the boiling coil 71, the liquid natural gas is passed by conduit 74 for flow through coil 75 immersed in a pool 76 of liquid methane at a temperature of about 250 F. contained in vessel 77. The liquid natural gas at a pressure of about 470 p.s.i.a. is further cooled to about 240" F. upon flowing through the coil 75 and is then conducted by conduit 78 to pressure reducing valve 79, where the pressure of the liquid natural gas is reduced to about 50 p.s.i.a., and then conducted by conduit 80 into the top of the stripping column 72. The column 72 provides a stripping zone containing conventional liquid vapor contact means such as bubbletype fractionating plates 81 and functions to remove substantially the total nitrogen content from the liquid natural gas as described in detail below. Top gas withdrawn from the column 72 through conduit 82 contains substantially the total nitrogen of the natural gas fed to the cycle and liquid forming the pool 73 comprises liquefied natural gas substantially free of nitrogen. Liquid natural gas, at a temperature of about 230 F. and a pressure of about 50 p.s.i.a. is withdrawn from the column 72 by conduit =83 and then subcooled to a temperature of about 248 F. upon flowing through coil 84 immersed in the pool 76 of liquid methane in the vessel 77. Thereafter, the subcooled natural gas is conducted by conduit 85, reduced in pressure in valve 86 to about p.s.i.a., and then introduced into storage tank 87 from which liquid natural gas may be withdrawn through valved conduit 88 as desired.

As mentioned above, one of the objects of the present invention is to provide a novel cascade refrigeration system employing a plurality of separate refrigerants to establish a plurality of progressively decreasing temperature levels for incremental cooling of a gaseous mixture to liquefaction temperature at relatively low pressure. The liquid glycol in heat exchange device =16, the liquid propane in vessel 25, the liquid ethylene in vessels 43, 57 and 61, and the liquid methane in vessels 65, 69' and 77 establish such progressively decreasing temperature levels, and the manner the liquid refrigerant is obtained at each temperature level is now described. With reference to FIGURE 1A, the glycol refrigeration system includes a pump 90, having an inlet connected by conduit 91 with the shell side of the heat exchange device 16, for circulating glycol, warmed upon heat exchange with the natural gas stream flowing to the passageway 15, through coil 92, immersed in the pool 24 of liquid propane in the vessel 25, and then by conduit 93 to the heat exchange device 16. The glycol leaves the heat exchange device 16 at a temperature of about 65 F. and is cooled to about 55 F. upon flowing through the coil 92.

Liquid propane forming the pool 24 in the vessel is produced in a closed refrigeration cycle including a heat exchange device 94 having passageways 95 and 96, a compressor 97, an aftercooler 98, and a pressure reducing valve 99. The vessel 25 is under a pressure of about 33 p.s.i.a. and propane vapor at about 7" F., produced upon vaporization of liquid propane upon flow of the natural gas through the coil 23, upon the flow of glycol through the coil 92 and upon the flow of ethylene through coil 100 as described below, is withdrawn from the vessel 25 through conduit 101, warmed to about ambient temperature upon flowing through the passageway 95, and then compressed to about 195 p.s.i.a. by the compressor 97. The compressed propane is cooled and lique fled at a temperature of about 94 F. in the cooler 98 and then subcooled to about F. upon flowing through the passageway 96. The liquid propane is then reduced in pressure in the valve 99 to about 33 p.s.i.a. and introduced into the vessel 25 at about -7 F.

As shown on FIGURE 1B, the ethylene refrigeration system includes a multi-stage compressor having a first stage 105, an intermediate stage 106 and a last stage 107, having an aftercooler, not shown, which delivers the ethylene under a pressure of about 425 p.s.i.a. and a temperature of about 94 F. in discharge conduit 108 communicating with passageway 109 at the warm end of a multipass heat exchange device 110. The compressed ethylene gas flows in countercurrent heat interchange with relatively cold ethylene vapor flowing through passageways 111, 112 and 113, described below, and leaves the passageway 109 at about 29 F. in conduit 114. The latter conduit conducts the cool ethylene gas for flow through the coil 100 immersed in the pool 24 of liquid propane 6 in the vessel 25 as shown on FIGURE 1A. The com pressed ethylene is totally liquefied in the coil and at a temperature of about 0 F. is returned by way of conduit 115 to the heat exchange device 110 for flow through passageway 116 in countercurrent heat interchange with relatively cold ethylene vapors mentioned above. The compressed liquid ethylene at about 400 p.s.i.a. and about 27" F. leaves the passageway 116 by conduit 117 and fed to pressure reducing valve 118 where the pressure is reduced to about 138 p.s.i.a. with a concomitant further cooling to a temperature of about 64 F., and then introduced by conduit 119 into the vessel 43 to provide the pool 42 of liquid ethylene at the temperature level of about -64 F. Ethylene vapor flashed in the valve 118 and ethylene vapor produced upon vaporization of liquid of the pool 42 by cooling the natural gas stream flowing through the coil 41 is withdrawn from the vessel 43 by conduit 120 and flowed through the passageway 113 of the heat exchange device 110 from which gaseous ethylene at a pressure of about 135 p.s.i.a. and at about ambient temperature is withdrawn by conduit 121 and fed to the inlet of the compressor 107. Liquid ethylene is withdrawn from the vessel 43 by conduit 122 and subcooled to about 86 F. upon flowing through passageway 123 of heat exchange device 124 in countercurrent heat interchange with relatively cold ethylene vapor flowing through passageways 125 and 126, as described below. The subcooled liquid ethylene is reduced in pressure in valve 127 to about 50 p.s.i.a., with a concomitant reduction in temperature to about -l12 F., and then conducted by conduit 128 into the vessel 57 to provide the pool 56 of liquid ethylene at the temperature level of about ll2 F. Ethylene vapor produced by flashing upon pressure reduction in the valve 127 and upon vaporization of liquid ethylene of the pool 56 by cooling the natural gas flowing through the coil 55 is withdrawn from the vessel 57 at a pressure of about 50 p.s.i.a. and a temperature of about ll2 F. by way of conduit 129 and conducted thereby for flow through the passageway 125 of the heat exchange device 124 and then by conduit 130 for flow through the passageway 112 of the heat exchange device 110 from which the gaseous ethylene, at about ambient temperature and a pressure of about 44 p.s.i.a., leaves the warm end of the heat exchange device by conduit 131 and fed to the inlet of compressor 106. The discharge of the latter compressor is provided with a conventional intercooler, not shown, and is conducted by conduit 132 to the inlet of the compressor 107. Liquid ethylene is withdrawn from the pool 56 of the vessel 57 by conduit 133 and subcooled to a temperature of about 124 F. upon flowing through passageway 134 of heat exchange device 135 in countercurrent heat interchange with relatively cold ethylene vapor flowing through the passageway 136, as described below. Such subcooled liquid ethylene, at a pressure of about 48 p.s.i.a., is fed by conduit 137 to pressure reduction valve 138 where the pressure is reduced to about 21 p.s.i.a. with a concomitant reduction in temperature to about -l43 F. and then passed by conduit 139 to vessel 61 to provide the pool 60 of liquid ethylene at the temperature level of about 143" F. Ethylene vapor at a temperature of about 143 F. and a pressure of about 21 p.s.i.a., which comprises vapor flashed in the pressure reduction valve 138 and vapor resulting from vaporization of liquid ethylene of the pool 60 upon cooling the natural gas stream in the coil 59 and upon cooling a methane stream flowing through coil 140 also immersed in the pool 60, described in detail below, is withdrawn from the vessel 61 by conduit 141 and flowed through the passageway 136 of the heat exchange device 135, then conducted by conduit 142 for flow through the passage-way 126 of the heat exchange device 124, and then conducted by conduit 143 for flow through the passageway 111 of the heat exchange device 110 from which the gaseous ethylene is withdrawn by conduit 144 at substantially ambient temperature and at a pressure of about 15 p.s.i.a. and fed to the section inlet of the compressor 105; the latter compressor has an intercooler, not shown, and the discharge thereof is conducted by conduit 145 to the inlet of the compressor 106.

The methane refrigeration system, shown essentially on FIGURE 10, includes a multi-stage compressor having a first compressor stage 150, an intermediate compressor stage 151 and a final compressor stage 152 from which gaseous methane under a pressure of about 515 p.s.i.a. and a temperature of about 94 F is discharged into conduit 153 provided with a conventional aftercooler, not shown. The discharge of the compressor 150 is fed by conduit 150A to the inlet of the compressor 151 and the discharge of the latter compressor is fed by conduit 151A to the inlet of the compressor 152, conventional innercoolers, not shown, being provided between the compressor 150, 151 and between the compressors 151, 152. The conduit 153 conducts the compressed gaseous methane for flow through passageway 154 of a multi-pass heat exchange device 155 including passageway 156 for relatively cold low pressure methane vapor, passageway 157 for relatively cold methane vapor under intermediate pressure, passageway 158 for relatively cold methane vapor under high pressure and passageway 159 for relatively cold vent gas. The heat exchange device 155 also includes passageway 160 for cooling the minor portion of the natural gas feed stream diverted at point 27 into the conduit 28 as described above. As shown on FIG- URE 1C, the conduit 28 is connected to the passageway 160 at the warm end of the heat exchange device 155 for flow of the minor portion of the natural gas feed stream in countercurrent heat interchange with the relatively cold vapor flowing through the passageways 156, 157, 158 and 159 to cool the substream of the natural gas feed to about 123 F., the cold end of the passageway 160 being connected to the conduit 29 for returning the thus cooled substream of natural gas feed for merger with the remaining natural gas feed at point 30 as described above. The compressed gaseous methane leaves the cold end of the passageway 154 at about 120 F. and is conducted by conduit 161 for flow through the coil 140 immersed in the pool 60 of liquid ethylene in the vessel 61, see FIGURE 1B. The cooled compressed gaseous methane is divided at point 162 within a major portion flowing through the coil 140 by way of conduit 163 and with the remaining portion being introduced by conduit 164 at point 165 into the stream of natural gas feed flowing in the conduit 58, a control valve 166 being provided in the conduit 164 to determine the quantity of gaseous methane introduced into the natural gas feed stream for a purpose that will be described below.

The compressed methane at a pressuer of about 525 p.s.i.a. is cooled to a temperature of about 137 F. and totally liquefied upon flowing through the coil 140 in heat interchange with liquid ethylene of pool 60 and is then conducted by conduit 167 for flow through passageway 168 of multi-pass heat exchange device 169 in countercurrent heat interchange with relatively cold vapors described below. The heat exchange device 169 includes a low pressure methane vapor passageway 170 connected to the passageway 156 of heat exchange device 155 by conduit 171, an intermediate pressure methane vapor passageway 172 connected to the passageway 157 of heat exchange device 155 by conduit 173, a high pressure methane vapor passageway 174 connected to the passageway 158 of heat exchange device 155 by conduit 175, and a vent vapor passageway 176 connected to the passageway 159 of heat exchange device 155 by conduit 177. While the heat exchange devices 155 and 169 are shown as separate devices, it is understood that both heat exchange devices may be embodied in a single jacket. The liquid methane is subcooled to about 162 F. upon flowing through the passageway 168 and is conducted by conduit 180 to pressure reducing valve 181 where the pressure of the liquid methane is reduced to about p.s.i.a. with concomitant cooling to about -196 F. and then conducted by conduit 182 to the vessel 65 to provide the pool 64 of liquid methane at the temperature level of about 196 F. Methane vapor flashed in the valve 181 and produced upon vaporization of liquid methane of pool 64 by cooling the natural gas feed flowing through the coil 63 is withdrawn from the vessel 65 by conduit 183 and flowed successively through passageways 174 and 158. The gaseous methane emerges from the warm end of heat exchange device 156 at substantially ambient temperature and under a pressure of about 135 p.s.i.a. and is conducted by conduit 184 to the inlet of the compressor 152. Liquid methane is withdrawn from the vessel 65 by conduit 185 and is subcooled to a temperature of about 220 F. upon flowing through passageway 186 of the heat exchange device 169 in countercurrent heat interchange with the relatively cold vapors flowing through the passageways 170, 172 and 176. Such subcooled liquid methane is conducted by conduit 187 to pressure reducing valve 188 where the pressure is reduced to about 55 p.s.i.a. with concomitant cooling of the methane to about 225 F. and then conducted by conduit 189 to the vessel 69 to provide the pool 68 of liquid methane at the temperature level of 225 F. Methane vapor comprising a. vapor flashed in the valve 188 and vapor produced upon vaporization of liquid methane by cooling the natural gas feed flowing through the coil 67 is withdrawn from the vessel 69 by conduit 190 and then passed successively through passageway 172 of heat exchange device 169 and passageway 157 of heat exchange device from which gaseous methane at about 50 p.s.i.a. and substantially ambient temperature is passed by conduit 191 to the inlet of the compressor 151. Liquid methane at a temperature of about 255 F. is withdrawn from the vessel 69 by conduit 192 and subcooled to a temperature of about 240 F. upon flowing through passageway 193 of heat exchange device 194 in countercurrent heat interchange with relatively cold methane vapor flowing through passageway 195 of the heat exchange device. Such subcooled liquid methane is conducted by conduit 196 to pressure reducing valve 197 where its pressure is decreased to about 20 p.s.i.a. with concomitant cooling to about -252 F. and then conducted by conduit 198 to the vessel 77 to provide the pool 76 of liquid methane at the temperature level of about 252 F. Vapor flashed in the valve 197 and liquid methane vaporized upon subcooling the liquid streams flowing through the coils 75 and 84 is withdrawn from the vessel 77 through conduit 199 for flow through the passageway 195 of the heat exchange device 194 and then conducted by conduit 200 for successive flow through passageways and 156 of heat exchange devices 169 and 155, respectively. Gaseous methane leaves the warm end of the heat exchange device 155 under a pressure of about 16 p.s.i.a. and at substantially ambient temperature and is conducted by conduit 201 to the inlet of the compressor 150.

Vapor in the storage tank 87 under a pressure of about 15 p.s.i.a. and at a temperature about 257 F. is withdrawn by conduit 210 and fed to a compressor 211 which functions to raise the pressure of the vapor to about 20 p.s.i.a., that is, to correspond substantially to the pressure of the methane vapor in vessel 77. Vapor from the compressor 211, at about F., is conducted by conduit 212 and merged with the low pressure methane vapor flowing through the passageway 170 of the heat exchange device 169, at an appropriate temperature level, and flowed therewith to the compressor 150. Methane vapor withdrawn from the storage vessel 87 provides makeup methane for the methane refrigeration system; however, ordinarily the quantity of withdrawn methane vapor exceeds makeup requirements and, in order to balance the system, a predetermined quantity of gaseous methane is fed by control valve 166 and conduit 164 into the natural gas feed at point 165 of conduit 58, see

9 FIGURE 1B. Top gas from the stripping column 72 is conducted by the conduit 82 for flow through the passageways 176 and 159 of the heat exchange devices 169 and 155,*{respectively, and such top gas at substantially ambient temperature is withdrawn from the cycle through conduit 215 for use as fuel gas.

As mentioned above, it is an object of the present invention to provide a novel processfor removing an undesirable component from a gaseous mixture during liquefaction by effecting at least partial liquefaction of the gaseous mixture and provide liquefied gaseous mixture containing at least substantially the total undesirable component of the gaseous mixturefollowed by stripping such liquefied gaseous mixture by a vapor containing a relatively small percentage of the undesirable component. Such an arrangement is employed in the cycle described above to effect substantially complete removal of the nitrogen content of the natural-gas prior to storage. With reference to FIGURE 10 of the drawings, the stream of liquefied natural gas which leaves the coil 67 of the vessel 69 in conduit 70 at apressure of about 480 p.s.i.a and a temperature of about 199 F. has the following approximate composition:

Component: Mol percent N 2.96 C 93.30 C 2.40 C 0.63 C; 0.28 0.07 C 0.004

Such liquefiied natural gas is subcooled to a temperature of about 225 F. upon flowing through the boiling coil 71 and is further subcooled to about 240 F. upon flowing through the coil 75 in heat interchange with the liquid methane at the temperature level of about 252 F. The liquefied natural gas leaving the coil 75 is deeply subcooled to a temperature below the bubble point of such composition at a pressure of about 50 p.s.i.a., the operating pressure of the column 72. As a consequence, the pressure reduction of the subcooled liquid natural gas in valve 79 is achieved in the absence of vapor being flashed, thus permitting the relatively low pressure liquid natural gas to be introduced into the top of the column 72 above the uppermost fractionating plate and thereby avoiding the necessity of providing a column with a scrubbing section and a refluxing condenser which increases capital and operating costs. The stripping vapor for the column 72 is obtained by partial vaporization of the liquid bottoms of the column 72 upon subcooling the liquid natural gas flowing through the boiling coil 71. Liquid withdrawn from the column 72 by conduit 83, comprising substantially the total natural gas entering the cycle, is of the following approximate composition:

The top gas leaving the column through the conduit 82 comprises about 10% of the natural gas in conduit 70 and is of approximately the following composition:

Component: Mol percent N 26.24 C; 73 .37

As mentioned above, the column 37 is employed to reduce the hexane content of the natural gas to a value below the solubility limit of hexane in liquefied natural gas, that is, to a value less than of 1% of the liquefied natural gas product. With reference to FIGURES 1A and 1B, the top gas withdrawn from the column 37 in conduit 40 is of the following approximate composition:

About 1.7% of such top gas is liquefied in the coil 41 providing a liquid, fed to the column by conduit 48, of the following approximate composition:

Component: Mol percent N 0.59 C 47.97 C 8.97 C 8.11 C; 12.09 C 18.78 C 3.45

and a vapor in conduit 47 of the following approximate composition:

Component: Mol percent N 3.06 0 93.30 C 2.53 C 0.66 c 0.30 C 0.08 C 0.004

The liquid fed to the column by conduit 48 is stripped by natural gas entering the column through conduct 36 of a composition set forth below:

Component: Mol percent N 3.06 C 93.20 C 2.55 0 0.68 C; 0.32 C 0.12 C 0.05

and the resulting liquid forming the pool 39 comprises about .25% of the feed in conduit 36 and is of the following composition:

Component: Mol percent N 0.56 C 44.26 C 7.03 C 6.24 C; 9.22 C 15.15 C 17.51

It is to be expressly understood that the process disclosed in the drawings and conditions set forth in the foregoing description are for purposes of illustration and changes may be made in the process and the various pressure and temperature values may vary within the scope of the present invention as well understood by those skilled in the art. Reference therefor will be had to the appended claims for a definition of the limits of the invention.

What is claimed is:

1. Method for liquefying a gaseous mixture for storage under relatively low pressure comprising the steps of providing compressed gaseous mixture,

passing compressed gaseous mixture successively in indirect heat exchange with a plurality of refrigeration zones at progressively decreasing temperature levels from a high temperature level of about 50 F. to a low temperature level below the bubble point of the gaseous mixture at the relatively low pressure,

and reducing the pressure of the gaseous mixture to the relatively low pressure while maintaining the mixture below its bubble point following heat interchange with the refrigeration zone at the low temperature level,

the plurality of refrigeration zones at progressively decreasing temperature levels being provided by a cascade refrigeration system employing a plurality of separate refrigerants having progressively decreasing normal boiling point temperatures,

at least one of said plurality of cascaded refrigerants being provided at a single pressure to establish one of the temperature levels and a plurality of the refrigerants being provided at at least three progressively decreasing pressures to each provide at least three of the temperature levels.

2. Method for liquefying gaseous mixtures for storage under relatively low pressure as defined in claim 1 in which the cascade refrigeration system employs four separate refrigerants having progressively decreasing normal boiling point temperatures,

in which the refrigerant having the highest normal boiling point temperature establishes the high temperature level,

in which the refrigerant having the first next lowest boiling point temperature establishes the next lower temperature level,

in which the refrigerant having the second next lower normal boiling point temperature is provided at a plurality of different pressures to provide a plurality of temperature levels,

and in which the refrigerant at the lowest boiling point temperature is provided at a plurality of different pressures to provide a plurality of temperature levels including the low temperature level.

3. Method for liquefying gaseous mixture for storage under relatively low pressure as defined in claim 1 in which the gaseous mixture comprises natural gas and in which the cascade refrigeration system employs glycol, propane, ethylene, and methane refriger-ants,

in which the glycol refrigerant is employed to establish the high temperature level,

in which liquid propane refrigerant is employed to establish the first next lower temperature level,

and in which the ethylene and methane refrigerants are employed to establish temperature levels corresponding to their normal boiling points as well as temperature levels corresponding to their boiling points at a plurality of higher pressures.

4. Method for liquefying gaseous mixtures for storage under relatively low pressure as defined in claim 1 in which a part of the gaseous mixture is cooled by heat interchange with the refrigerant of the cascade system having the lowest boiling point.

5. Method for liquefying gaseous mixture for storage under relatively low pressure as defined in claim 1 in which the gaseous mixture comprises natural gas,

and in which the cascade refrigeration system employs liquid ethylene and liquid methane refrigerants each provided under a plurality of different pressures to provide a plurality of progressively decreasing temperature levels including the low temperature level.

6. Method for liquefying gaseous mixtures for storage under relatively low pressure as defined in claim 5 in which the compressed natural gas is partially liquefied upon heat interchange with ethylene refrigerant under high pressure,

and in which the liquefied portion of the natural gas is stripped to remove substantially the total hexane from the natural gas. 7. Method for liquefying gaseous mixtures for storage under relatively low pressure as defined in claim 5 in which compressed natural gas is liquefied and subcooled upon heat interchange with liquid methane refrigerant under low pressure to a temperature below the bubble point of natural gas at the pressure of an ensuing stripping step, and in which subcooled natural gas is stripped in a zone by a natural gas vapor substantially free of nitrogen. 8. Method for liquefying gaseous mixtures for storage under relatively low pressure as defined in claim. 7

in which the compressed gaseous mixture is passed in heat interchange with liquid natural gas free of nitrogen to aid in cooling the compressed natural gas prior to the stripping step and to provide the natural gas vapor substantially free of nitrogen. 9. Method of liquefying natural gas for storage under low pressure comprising the steps of compressing natural gas containing nitrogen to a relatively high pressure, passing the compressed natural gas successively in indirect heat exchange with a plurality of refrigerants at progressively decreasing temperature levels including a first lowest temperature level to liquefy the compressed natural gas, passing the liquefied compressed natural gas in indirect heat exchange with another refrigerant at a second lowest temperature level below the first low temperature level to further cool the liquefied compressed natural gas to a temperature below the bubble point of the natural gas at a relatively low predetermined pressure, reducing the pressure of the further cooled liquefied natural gas to the low predetermined pressure while maintaining the liquefied natural gas below its bubble point, and subjecting the further cooled liquefied natural gas under the low predetermined pressure to a stripping process with natural gas vapor essentially free of nitrogen to provide liquefied natural gas essentially free of nitrogen. 10. Method of liquefying natural gas for storage under relatively low pressure as defined in claim 9 in which the stripping process includes the steps of feeding the further cooled liquefied natural gas at said low predetermined pressure to the top of a confined zone, collecting a pool of liquefied natural gas essentially free of nitrogen in the bottom of the confined zone, passing liquefied natural gas in heat exchange with the pool of liquefied natural gas essentially free of nitrogen collecting in the zone to provide natural gas vapor essentially free of nitrogen, and withdrawing vapor containing nitrogen from the top of the confined zone. 11. Method of liquefying natural gas as defined in claim 9 in which the plurality of refrigerants are provided by a cascade refrigeration system employing a plurality of separate refrigerants having progressively decreasing normal boiling point temperatures, and in which the refrigerant at the first lowest temperature and the refrigerant 'at the second lowest temperautre level are provided by a separate refrigerant of the cascade system under different pressures. 12. Method of liquefying natural gas as defined in claim 11 including the step of passing liquefied compressed natural gas in heat interchange with liquefied natural gas essentially free of nitrogen before passing liquefied compressed 13 naturalgas in heat exchange with the refrigerant at the second lowest temperature level. 13. Method of liquefying natural gas as defined in claim 12 including the step of passing liquefied natural gas essentially free of nitrogen in heat exchange with the refrigerant at the second lowest temperature level.

5/1968 Becker 6227 XR 5/1954 Miller.

Tung 6240 XR Swenson et al 6240 XR De Lury et al 6240 XR Grunberg et a1 629 Maher et al.

Feist et al. 62--40 XR C'arr 6223 NORMAN Y U DKOFF, Primary Examiner.

10 V. W. PRETKA, Assistant Examiner.

U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington, 0.6. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,440,828 April 29, l9(

John A. Pryor et al.

, It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, lines l3, 19, 21, 22 and 47, "liquified", each occurrence,

should read liquefied line 48, "feasilbilty" should read feasibili Column 2, line 7, "capiatl" should read capital Column 7, line 45, "within" should read with line 53, "pressuer" should read pressure line 73, 162 F. should read -l72 F. Column 8, line 35, "255 F. should read 225 F. Column 10, line 39, "conduct" should read conduit Column 12, line 68, "temperautre" should read temperature Signed and sealed this 14th day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents 

